An electrochromic optical shutter ("EOS") is a thin layered structure which, upon the application of an electrical current or voltage, either momentary or sustained, reversibly changes its transmission of light. A thermochromic optical shutter ("TOS") is a layered structure whose transmission of light changes reversibly in response to temperature. A TOS is made from a layer of thermochromic material, which is a material whose light transmission changes reversibly with temperature.
EOS have been developed for use in glazings where it is desirable to control the transmission of light either for the sake of privacy or to control solar thermal gains or glare. For example, in a conference or bath room with glazings it is desirable to be able to prevent viewing through windows when privacy is required. Using a switch to turn on an EOS is an alternative to drawing curtains. An EOS controlled by a thermostat is an alternative to a TOS to prevent a solar collector from overheating when solar heat is not needed. A skylight whose light transmission is controlled by a light meter or a switch is a way to prevent glare when illumination is not wanted.
The means for varying the amount of sunlight admitted into buildings includes mechanical optical shutters, such as curtains, Venetian blinds, seasonal paint for greenhouses, and skylids. Skylids are automatic mechanical shutters which are actuated thermally by sunlight. Other examples of optical shutters are thin layers of liquid crystals in wrist watches and other displays, electrochromic coatings for automobile rear view mirrors which turn dark electronically, Kerr electro-optic cells for laser modulation, and photochromic glass for eyeglasses.
For many applications, optical shutters should not become highly absorptive of light when they are in their non-transmissive state. For windows and skylights, for example, it is preferred that incident sunlight is reflected rather than absorbed so that it does not become a heat load for the building. Light absorption is a problem with the above mentioned photochromic glass, and for photochromic, electrochromic, and thermochromic coatings, which all turn dark.
EOS have been developed where absorptivity and reflectivity are controlled by an electrical current which drifts an ionic dopant, such as H, Li, or Na, into a semiconductor such as WO.sub.3. EOS based on semiconductors have several disadvantages. They do not turn completely opaque, and thus do not offer complete privacy. When switched on, they become dark semi-silvered mirrors through which some view is possible, even though the view is obscured. Because they are partially mirrors, they can reflect an image of the occupants of the room, which is esthetically objectionable. Because they become partially absorptive, they are not completely effective in rejecting unwanted heat from sunlight. Their minimum transmission can be as high as 40%. The best minimum transmission is about 15%. Mass production is problematic because they require at least four layers, all of which must be made with extreme precision by high technology coating methods such as sputtering and coating in atmospheres of less than 1% relative humidity with toxic materials.
Liquid crystal optical shutters are equally complex; they can require two electrode layers, two polarizer layers, two adhesive layers, a polymeric medium, and liquid crystal particles, which degrade in sunlight. Some view is possible through liquid crystal optical shutters in their opaque state. TOS and photochromic optical shutters are activated by heat or light, respectively, and so do not have the advantage of being controlled manually by a switch or automatically by an electronic sensor.